The present invention relates to a diesel engine for a vehicle, and particularly relates to a diesel engine with a comparatively low compression ratio in which its geometric compression ratio is set within a range of 12:1 to 15:1.
In diesel engines that are supplied with fuel, of which a main component is diesel fuel, into cylinders and perform combustion by the fuel being compressed to cause a self-ignition, a comparatively low compression ratio in which a geometric compression ratio is, for example, 15:1 or lower is set in order to achieve, for example, a reduction of a discharge amount of NOx. That is, the low compression ratio reduces a speed of the combustion in each of the cylinders and suppresses generation of NOx (see JP2008-261236A). Further, because the lowered compression ratio of the engine reduces a mechanical resistance loss, it is also beneficial in improving a thermal efficiency of the engine.
However, when the geometric compression ratio of the diesel engine is set low, a temperature of the engine at the end of compression stroke is decreased corresponding to the low compression ratio. Therefore, self-ignition conditions become difficult to be met within, for example, an operating range where an engine load and an engine speed are low. Further, in particular, in the diesel engine disclosed in JP2008-261236A, an ignition performance degrades as a cetane number becomes lower depending on a property of fuel to be supplied. Therefore, when these factors relating to the operating conditions of the engine and the property of the fuel are combined, the self-ignition conditions further become difficult to be met. In association with the above, JP2008-261236A discloses a control of improving a starting performance of the engine by estimating the cetane number of the fuel when the engine is started and, according to the estimation result, changing a temperature of a glow plug and a fuel injection timing.
Further, JP2007-040218A discloses an art for increasing, when a compression ratio of a spark-ignition engine, which performs ignition combustion on the compression stroke, decreases due to a degradation of a compression system thereof, an internal EGR amount by advancing closing timings of exhaust valves, and thereby, increasing a temperature inside cylinders to secure ignitability therein.
As described above, in achieving the lowered compression ratio with the diesel engine, surely securing the ignitability of the fuel regardless of the operating conditions of the engine and the property of the fuel to be supplied is important.
The present invention is made in view of the above situations and surely secures a fuel ignitability in a diesel engine particularly with a low compression ratio, which is for mounting on an automobile.